1. Field of the Invention
This invention generally relates to a method and apparatus that provide the basic conditions for thermal treatment of carbonaceous materials in a moving bed reactor with geometry that allows the reactor to also serve as a moving bed filter, and thereby minimize particulate entrainment in the gas stream from the reactor.
2. Description of the Related Art
The thermochemical conversion (e.g., carbonization, gasification, pyrolysis) of biomass and other carbonaceous materials (e.g., peat, coal, tires, plastics) can produce solid, liquid, and gaseous products which can be used in a variety of energy and chemical production applications. These processes are relatively fast; thus large amounts of feedstock can be converted quickly in a relatively small footprint. Due to these and other advantages, coupled with the widespread availability of carbonaceous materials for feedstocks, these processes have become important to meet the growing worldwide use of energy and chemicals.
In the following discussion the term “gas stream” is used to refer to streams that may contain gas and vapor, as well particulates (which as used herein particulates refers to droplets and solid particles) and other materials. Likewise, the term “gas” as used herein can include gas, vapor, particulates, aerosols, and other materials. The term “thermal treatment” is used to refer to carbonization, gasification, pyrolysis, liquefaction, or other related thermochemical reactions and processes, and recognize that a fast pyrolysis process is a specialized method of gasification. It should also be noted that although the present disclosure focuses upon biomass processes and fast pyrolysis processes, it is appreciated the technology embodied herein may be used in conjunction with other processes and gas streams.
Although apparatuses to perform fast pyrolysis processes can vary widely, the basic conditions required for fast pyrolysis are well known worldwide as described in U.S. Pat. No. 5,961,786. These basic requirements include:                1) An enclosed reactor to provide a reaction environment in the relative absence of oxygen,        2) A very rapid feedstock heating rate, which can be as high as 1,000,000° C. per second,        3) A controlled, elevated reaction temperature typically in the range of 350 to 800° C.,        4) A controlled, short reaction/residence time which is typically in the range of 0.03 to 2 seconds,        5) A rapid quench of the vapor, typically cooled below 350° C. within 2 seconds, to minimize time for secondary reactions to occur which would decrease liquid product yields and potentially create undesirable products.        
To contrast, slow pyrolysis processes will have relatively higher gas and char yields and, when made from wood feedstocks, produce a two-phase, highly viscous oil/tar. Conversely, a well-designed fast pyrolysis system processing wood feedstocks will have a relatively high liquid yield and relatively low char and gas yields. Additionally, the fast pyrolysis of wood feedstocks should produce a single phase, relatively low viscosity liquid. Although wood is used herein as an example feedstock, other carbonaceous feedstocks can also be used.
A variety of different reactor systems have been researched for fast pyrolysis applications. As described in U.S. Pat. No. 5,961,786, these include cyclonic ablative reactor, vacuum, auger, fluidized bed, transport bed, and moving bed pyrolysis reactors.
Cyclonic Ablative Reactors
Feedstock particles are injected into cyclonic reactor systems so that the particles travel around the heated surface of the cyclone whereby they are ablatively heated and eventually vaporized. The vapors are immediately carried to a quenching device. This approach works well at a small scale, but is restricted because of the limited heat transfer rate through a reactor wall and complexity associated with scale up.
Vacuum Pyrolysis
Vacuum pyrolysis systems use a vacuum to quickly remove vapors from the surface of the reacting feedstock particles. This immediate vapor removal mitigates the need for very rapid heat transfer. Vacuum processes suffer from their high parasitic load requirements, the inherent difficulty associated with scale up, the potential for inadequate solids flow, and the general lack of demonstrated chemical conversion processes at an industrial scale.
Auger Reactors
Auger reactors typically have an auger or augers inside a horizontal cylinder or trough to convey the feedstock and circulate the feedstock against the hot cylinder wall where the feedstock can be ablatively heated. Thus auger reactors are relatively simple, inexpensive devices.
Without some type of heat carrying medium inside the reactor, auger reactors are limited in size since the reactor wall provides a limited surface area for heat transfer. The addition of fins or other protrusions, or the use of hollow heated augers does not overcome these heat transfer limitations enough to allow large scale industrial application.
Where a heat carrier is employed, research at Iowa State University has suggested heat carrier mass to feedstock mass ratios in the range of 20:1 are reasonable, which is significantly lower than the ratios typically specified for transport bed reactors.
Fluidized Bed Reactors
Fluidized bed reactors employ a bed of inert, relatively small particles in an enclosed vertical vessel that is fluidized by blowing a gas through the bed from below. The reactor bed may be heated by the fluidizing gas stream, tubes inside the bed, indirectly from the outside, or other means. Pyrolysis is achieved by direct heat transfer to the feedstock particles from the fluidizing gas and from ablation with the bed particles.
Although fluidized bed fast pyrolysis reactors achieve rapid heating rates and a controlled elevated temperature, they are limited by relatively long residence times that are beyond the optimal required for maximum yields of liquids and certain valuable chemicals. Major limitations of fluidized bed reactors are the relative high difficulty to scale them up for industrial applications and the high energy requirements for fluidization. The heat carrying capacity of a gas is also limiting.
Transport Reactors
Transport reactors are configured similar to fluidized bed reactors with a mixing zone that is analogous to the bed in a fluidized bed reactor. These reactors are defined according to the nature of the transporting medium, which can be a non-oxidative transport gas or non-oxidative transport gas plus solids, and by the direction of flow through the reactor, which can be downflow or upflow.
In order to achieve the high heat transfer rates required for fast pyrolysis, transport bed reactors usually use a solid form heat carrier to supplement the heat in the transporting gas. Typically these heat carrying solids are inert silica sand or alumina-silica catalyst with a mean particle size in the range of 40 to 500 microns. Particles in this size range for sand have individual particle densities light enough to allow transport through the transport bed and heat carrier circulation system. However, the use of solid heat carrying solids in this size range makes the physical separation of the sand heat carrier and fine char particles generated by the process impossible. Therefore the char is typically burned as part of the fast pyrolysis process to provide thermal energy for the process and an ash residue generated which must then be removed by some means and disposed. Thus char recovery is not an option with these systems, and the loss of the char as a product can be a major economic drawback.
Additional major drawbacks associated with transport reactor systems include poor mixing of feedstock and heat carrier, essentially no particle ablation, poor heat transfer to the reacting particles, and high parasitic energy requirements. Therefore these systems also have limitations as to the potential to achieve high liquid yields and desired chemicals.
In an attempt to mitigate these limitations for transport bed reactors, a solid organic heat carrier has been used as is disclosed in U.S. Pat. No. 4,153,514, where hot char was used as a heat carrier. The organic heat carrier of the U.S. Pat. No. 4,153,514 does not provide the thermal, physical, and chemical properties required for effective fast pyrolysis reactions. The char material does not provide the heat demand, surface area for intimate contact, the rapid heat transfer between the heat carrier and feedstock, and the physical integrity for efficient and practical pyrolysis. Furthermore, the char participates in the reaction and is thus consumed and converted into undesirable side products as it passes through the reaction zone, thus a diminishing quantity of heat carrier is available as the char proceeds through the reaction zone. More importantly, due to the characteristics of char, it is impossible to achieve a sufficiently high ratio of heat carrier to feedstock to achieve fast pyrolysis.
U.S. Pat. No. 4,153,514 specifies the sand heat carrier mass to feedstock mass to range from 12:1 to 200:1 in order to obtain the desired heat transfer rates and feedstock residence times. Thus the parasitic loads to move the transport gas and inert solids through the bed can be quite high relative to most other types of fast pyrolysis reactors.
Moving Bed Reactors
Moving bed reactors are similar in design to transport bed reactors but use solid heat carriers for transferring heat to the feedstock particles. Although pneumatic methods can be used, mechanical means are more typically used to withdraw bed particles from the reactor, circulate bed particles outside the reactor to reheat the media and remove char, and reinject the bed particles into the reactor. Within the reactor gravity or mechanical means may be used to accomplish bed particle mixing and movement, which is typically downward.
Table 1 provides a comparison between two different kinds of heat carriers. Sand is commonly used in transport reactors and steel shot and other media have been used in moving bed reactors.
TABLE 1Comparison of two different types of heat carrierHeat Capacity,Bulk Density,Heat Capacity,Heat CarrierBtu/lb-Flb/cfBtu/cf-F.Sand, dry0.19188-10016.8-19.1Stainless Steel shot,0.1128030.80.125″ dia
In addition to the higher heat transport capability provided by relatively heavy media such as steel shot in moving beds, moving bed reactors have the advantages of significantly lower parasitic energy loads, simplicity of operation, and—depending partially on the choice of heat carrier—relatively low heat carrier mass to feedstock mass ratios.
Reactor Review Summary
A practical, economical, commercial thermal treatment process requires:
1) Relatively high yields of the liquid products,
2) Scalability of the process to industrial size,
3) Technical and business feasibility for industrial use.
In summary, a moving bed reactor can provide a method and apparatus with an inorganic heat carrier with its inherent ablative heat transfer properties for thermal treatment, including providing conditions for true fast pyrolysis processing and products therefrom.
In particular, the moving bed system and process provides a system that combines adequate reactor temperature, short residence time and rapid product cooling to achieve true fast pyrolysis with the required aspect of extremely rapid heat transfer to the feedstock particles. In order to effectively achieve sufficiently high heat transfer rates in a reactor system, direct ablative contact between the solid heat carrier (that is, the reactor wall or solid particles) and the feedstock is required. This type of heat transfer can only be accomplished in a reactor system if that system provides a relatively large amount of hot surface area to the reacting biomass, per unit time, per unit volume of reactor. If the heat transfer surface is relatively small, either the rate of heat transfer is reduced or the reactor can only process a relatively small amount of material per unit time per unit volume of reactor (with a significant negative effect on the economics of the reactor). Thus, the moving bed reactor fulfills the need for a system that utilizes the high bulk density of an inorganic heat carrier to allow a high loading (mass of heat carrier to mass of feed) to be achieved in a relatively small reactor volume in order to provide a very large heat transfer surface to be available to the feed.
The reactor of the present invention, which is described below in detail, meets these requirements and improves on prior art thermal treatment systems and reactors by providing a relatively simple, low cost and effective combination moving bed thermal treatment reactor system and moving bed filter.
One long-time major hurdle for thermochemical processes is the cleaning of contaminates such as tars and particulates from the gas and vapor product streams. Cleanup of gas streams from these thermochemical processes is important to prevent plugging and fouling of ducting, piping, and devices downstream of the gasification reactor. These contaminants may also poison or otherwise interfere with the operation of devices or other processes downstream of the reactor.
This cleanup is compounded by the temperatures of the gas and vapor streams, which can exceed 900° C. for thermochemical processes. Cleanup can be also compounded by the presence and stickiness of tars in the gas stream, especially if char particles are present, as char particles will increase the rate of buildup on ducting and other surfaces and can, under some circumstances, provide a catalytic effect.
What is considered an acceptable level of gas stream contamination may vary depending on the end use of the gas and vapor. For example, Basu in Biomass Gasification and Pyrolysis, Practical Design and Theory indicates that catalysts and fuel cells require very clean gas streams (0.02 mg/Nm3 for particulates and 0.1 mg/Nm3 for tar) to prevent binding, poisoning, or other interference with their operation. Treatments by catalysis are widely considered key processes for upgrading gas and liquid products from thermal treatment processes, thus solving the hot gas cleanup issues would greatly facilitate the commercialization of catalytic processes for gas and vapor upgrading purposes.
In the case of bio-oil vapors derived from a thermal treatment process, particulates in the gas/vapor stream can be composed of char, which contains the ash from the biomass. A portion of the ash contains mineral content, which can react with the vapor and reduce the final bio-oil yield. Therefore, all other things equal, decreasing the amount of char particulates in the gas stream will decrease these secondary reactions.
Over time, various hot gas cleanup methods have been developed and some are commercially available. Cyclones and swirl tubes have been used for particulate cleanup in gas streams with partial success. These devices work by imposing an artificial gravity field in the form of a centrifugal force on the gas stream particles.
The capture efficiency of well-engineered cyclones decreases rapidly for particles below 5 microns in size, with typically two or more well-designed cyclones in series required to achieve a filtration efficiency approaching 99% at the 5 micron particle size level. However, particulates in the gas stream can be as small as 0.1 micron, thus cyclonic devices alone will not achieve the necessary levels of particulate cleanup.
Other examples of commercial hot gas clean up technologies include ceramic filters and sintered metal filters. These filters operate in a manner similar to baghouses that are commonly used for control of fine particulates in gaseous emissions in industry. In practice, the gas stream is channeled through the filter material and the particulates are removed by passage through small pores in the filter that results in the buildup of a filter cake, which provides a further filtering action. Usually the filters must be periodically taken offline and the filter cake removed, typically by back flushing with a gas stream, or cleaning with chemicals, or other means. Because the ratio of pore area to surface area is relatively small, these filters require very large surface areas and, in the case of ceramic filters, can be fragile. In addition to their massive physical size, these filters are also very expensive; the capital cost of a ceramic or sintered metal filter system can equal the entire capital cost of the balance of the thermochemical process system.
Electrostatic precipitators (ESPs) are effective at removing aerosols and particulates that will take an electrical charge. These devices can also operate under high temperature conditions. Unfortunately, some of the thermal treatment liquid product in the gas stream can be in the form of aerosols and would be lost through the use of an ESP placed within or immediately after the reactor to remove gas stream particulates.
Prior art for gas cleanup includes fixed bed filter systems. An example of a fixed bed filter system is U.S. Pat. No. 4,744,964, which utilizes an agitated fixed bed of granular material to “purify” pyrolysis gases with simultaneous neutralization and dust separation. This method has trouble removing the filter cake effectively, even with the agitation.
Moving bed filters (MBFs) have been used with some success in removing particulates from gas streams from thermal treatment processes, including fast pyrolysis processes, outside the reactor, particularly when the MBFs are preceded by cyclones to remove most of the particulates before the MBF. The concept of the MBF as described further in U.S. Pat. No. 7,309,384 is to provide a self cleaning mechanism for the filter, so that the system can operate with significantly less down time. MBFs have a bed of moving filter material, which is typically some form of small aggregate that flows downward by gravity within an enclosed vessel housing the filter. Gas enters the filter either from the top, bottom, or side and typically exits opposite the gas' entry point. Solid gas contaminants captured in the filter bed are swept along with the filter material and are removed from the bed material externally in a separate operation, and the filter bed material recycled back to flow through the MBF again.
The choice of bed particle materials for MBFs is important as bed particles with low densities will become fluidized and, in the worst case, entrained in the exiting gas stream at high gas flowrates and thus limit the throughput and efficiency of the MBF. This limitation has been addressed by various methods, including using a screen against the top of the bed to keep the bed from fluidizing as described in U.S. Pat. No. 7,309,384.
U.S. Pat. No. 7,309,384 indicates that excessive gas pressure drop is a problem because of the small area available for gas exiting the filter and the gas pathway through the MBF. The method described thus requires special measures to mitigate gas pressure drop through the device.
Unless tar removal is the goal, all gas cleanup devices must be kept at temperatures above the dew point of tars and vapors to prevent them from condensing and building up on surfaces. In addition to insulation, and depending on their location in the process, the gas cleanup devices may require a heat source(s) and method of transferring heat to the devices. This requirement adds to the complexity and cost of the cleanup train, increases the physical size of the system, and in the case of portable systems, can add significant weight to the apparatus.
In summary, the traditional MBF as a separate operation downstream from the reactor, must have a mechanism for keeping the device hot, must have a mechanism for removing the filtered materials from the bed materials (usually conducted in a separate, external operation or by taking the device offline periodically), must have a mechanism for recycling the bed material back to the top of the MBF, and must have a separate mechanism for controlling bed depth.
All of the gas cleanup methods and apparatus discussed apply to gas cleaning systems downstream of the reactor—in other words—external to the reactor. External gas cleanup devices increase the length of the gas path to the liquid recovery systems, which increases gas pressure drop and increases the time for gas passage, resulting in more time for undesirable secondary reactions to occur with the vapor, resulting in reduced liquid yields and other problems.
A method whereby a hot gas cleanup system was built into a reactor would have several advantages. One attempt at combining hot gas cleanup system into a reactor is disclosed in U.S. Pat. No. 4,151,044 where a fast pyrolysis reactor is built into a cyclone. Fast pyrolysis reactors built in the form of cyclones have serious limitations since they must perform at least two tasks simultaneously—holding the pyrolysis reaction and separation of solids. Typically these two operations have their own optimal conditions and a compromise must be found. Therefore the overall system efficiency is compromised.
U.S. Pat. No. 7,202,389 describes a combined fast pyrolysis reactor and gas filtration system in an attempt to overcome these limitations by mounting a rotating filter directly on or in the gas exits of the cyclone. Thus the exiting gas is forced through the filter and particulates captured and removed from the gas stream. The rotating filter is cleaned by means of a fluid jet blowing in a reverse flow direction to the gas stream.